A circuit arrangement is known from DE 38 40 845 A1 and also from DE 40 05 850 A1.
In this circuit arrangement there is a (DC) voltage source with two terminals, of which typically one is a ground terminal. The low-pressure discharge lamp is operated on an AC voltage. In order to generate an AC voltage switches are provided which are driven by suitable means for controlling the switches in such a way that at least one electrode of the low-pressure discharge lamp is coupled alternately to one terminal and the other terminal. In order to enable operation including starting in the low-pressure discharge lamp, a series resonant circuit is provided. Said series resonant circuit includes an inductive element which is connected in series with the low-pressure discharge lamp, i.e. is coupled with one terminal to the electrode of the low-pressure discharge lamp. The series resonant circuit furthermore includes a capacitive element or else a plurality of such capacitive elements, the at least one capacitive element being connected in series with the inductive element, to be precise in parallel with the low-pressure discharge lamp.
DE 38 40 845 A1 describes how gentle starting of the lamp is made possible: A circuit point of the series resonant circuit is connected to a PTC thermistor, and the PTC thermistor is coupled, via a diode, to the first terminal of the voltage source. In the example from DE 38 40 845 A1, the PTC thermistor is at the same time coupled, via a second diode, to the second terminal of the voltage source. For the purposes of the circuit, in principle one of the two diodes, e.g. the diode which is coupled to the ground terminal, is sufficient. The voltage present at the low-pressure discharge lamp is clamped by the diodes, i.e. only voltages which are lower than the voltage present at the voltage source are present across the low-pressure discharge lamp. These voltages are insufficient for starting. By virtue of the means for controlling the switches, the series resonant circuit is excited if it is also not yet at resonance. As a result, the electrodes of the low-pressure discharge lamp are preheated. At the same time, the PTC thermistor is heated. As soon as the PTC thermistor has a high resistance value, it is possible for there to be a higher voltage drop across the low-pressure discharge lamp than is present at the voltage source. The series resonant circuit enters resonance and there is a voltage drop which is sufficiently high for starting, a starting voltage, across the low-pressure discharge lamp. After starting, the voltage drop across the low-pressure discharge lamp again falls below that which is present at the voltage source. The PTC thermistor is then cooled down again, but during conventional operation there is no longer a current flowing via said PTC thermistor.
Anomalies may occur during operation of a low-pressure discharge lamp. The low-pressure discharge lamps demonstrate an excessively high lamp voltage in the case of some anomalies. These result in a high lamp power, and the increased lamp power in turn results in overheating of the ballast of the low-pressure discharge lamp, possibly also in local overheating of the low-pressure discharge lamp itself. The overheating brings about a hazardous situation.
An excessively high lamp voltage occurs in particular at the end of life of the lamp, possibly also in the event of contamination of the lamp. A hazardous situation can also occur in the event of a lamp with an excessively high power erroneously being fitted.
In order to avoid hazards, the new approach of detecting the lamp voltage has now been adopted. For this purpose, an additional winding can be provided on the lamp inductor, and an evaluation network is connected downstream of said additional winding. Likewise, capacitive coupling-out from the series resonant circuit can also take place and an evaluation network can be connected downstream.
A high degree of complexity is involved in the detection of the lamp voltage.